Arc discharge ballast suitable for automotive applications

ABSTRACT

A circuit for starting and operating a high intensity discharge (HID)lamp in a dc mode includes a pulse width modulator (PWM) coupled to first and second direct current input terminals. The PWM includes a high frequency generator driving a pair of semiconductor switches. A first transformer having primary and secondary windings is coupled to the semiconductor switches. A rectifier and voltage multiplier circuit converts the voltage across the secondary winding of the first transformer to a direct current voltage. A first energy storage circuit coupled to the rectifier and voltage multiplier circuit stores energy for generating a high voltage spike which initiates a glow condition in the lamp. A secondary energy storage circuit, which stores energy for application to the HID lamp during starting, insures proper glow-to-arc transition. The starting and operating circuit further includes a second transformer having first and second secondary windings coupled to the HID lamp and a pulse winding coupled to the first energy storage circuit. Preferably, the windings of the second transformer are adapted to apply a negative-going pulse across the lamp. The application of a negative-going pulse prevents failure of the quartz region of the glass around the lamp cathode by insuring that electrons properly migrate from the cathode to the anode. Moreover, the use of the negative-going pulse improves hot restriking.

CROSS-REFERENCE TO A RELATED APPLICATION

This application discloses, but does not claim, subject matter which isclaimed in U.S. Ser. No. 07/541,878 filed concurrently herewith andassigned to the Assignee of this application.

FIELD OF THE INVENTION

This invention relates to electrical circuits for starting andrestarting high intensity discharge (HID) lamps under either cold or hotconditions and for operating the lamps with direct current (dc) afterthey have started. This invention is particularly useful with miniaturemetal halide lamps intended for use in automotive applications, such asheadlights.

BACKGROUND OF THE INVENTION

It is well known that HID lamps commonly operate in three modes, i.e., acold or starting mode, an operating or steady state mode and a hotrestrike mode. During the cold or starting mode, a relatively high valueof ac or dc type starting or ignitor voltage (e.g., 25 KV) is appliedacross the lamp electrodes to first place the gasses of the lamp into asuitable ionized condition for striking or initiating a glow breakdownstate. This glow breakdown state which consists of an early glow,Townsend glow and anamolous glow, is followed by a delay time period toallow a transition into an arc condition between the electrodes of thelamp during the glow-to-arc state.

The initial or early glow state is characterized by a very low currentdensity and relatively high average energy per particle (i.e., E/N,wherein E is electric field and N is the number density of gas atoms inthe discharge). Generally the total pressure is fairly low beingdetermined by the rare gas R_(g) starting gas and the partial pressureof mercury is quite low, being determined by the ambient temperature.Since the excitation cross-section for e⁻ +R_(g) is smaller than for e⁻+Hg in the early stages of glow, the electrons undergo elasticcollisions with R_(g) and gain considerable energy until they eitherexcite R_(g) or Hg. The Hg excitation is more likely to lead toionization which produces an additional electron-ion pair. The majorityof the collisions impart low energy transfer or gas heating.

As the early glow state proceeds to the Townsend glow state, the currentincreases but the current density remains low. During this state, thesupply of electrons originate from surface field emission which occurswith a relatively high field strength, i.e., greater than 20 volts. Thisperiod is particularly damaging to the tungsten electrodes since anyionized gas atoms are driven back to the cathode with large energy whichpromotes sputtering of electrode material. Thus, it is important toestablish a thermionic or "hot-spot" mode of operation as quickly aspossible.

The glow current will continue to increase if a sustaining voltage (notthe ignitor voltage) is maintained. Usually, this initial sustainingvoltage is hundreds of volts, i.e., 1000 volts/cm field strength.

Near the cathode, current continuity must take place, i.e., ions arecollected and electrons emitted. However, the electrons move much morefreely so that within some distance from the cathode a space charge willexist, i.e., the ions cannot get to the cathode surface as fast as theelectrons can move away from the surface. This condition produces a"virtual" anode. The distribution of ions will be diffuse only becausethe charge density is low, i.e., the vast majority of the atoms areneutral. As an example, a fully developed discharge may only have localcharge densities of 10¹⁵ /cm³ while the total number density would be10¹⁸ /cm³. During the glow state, the density will be orders ofmagnitude smaller than the arc mode.

The diffuse "cloud" or sheath will shrink or move closer to the cathodeas the current increases, in part due to increased ionization and due toincreased repulsion within the sheath, i.e., the field gradient willincrease as the sheath shrinks.

Eventually, the potential generated by the cloud is large enough topermit fairly high energy collisions of the ions with the cathodesurface and upon shrinking of the sheath, the cathode becomes hot at afairly localized point. During this mode, called the anamolous glow, thecurrent reaches nearly the arc discharge level or greater, but thecurrent density is still fairly small and the field strength in thevicinity of the cathode surface is high.

As the electrode surface temperature increases, the liberation ofelectrons becomes easier due to the reduction in the barrier potentialof the electrode material. The production of electrons is stillcontrolled by the surface area and the electrode is said to be a coldemitter.

At some point, the electrode will emit electrons by virtue of its hightemperature, the so-called thermionic mode. This mode permits may timesthe current density so that the sheath collapses to a small spot closeto the surface of the cathode. In addition, the potential drops to 10-15volts depending on the material properties of the electrode. Thethermionic mode still requires some potential gradient to removeelectrons and this potential is referred to as the work function.

So during the glow, the potential across the electrodes or gap can bemaintained at hundreds of volts with the majority of the drop occurringaround the cathode and little field strength in the remaining region ofthe electrode gap. However, when the spot is formed, producingglow-to-arc, the voltage across the gap is controlled by the electronand ion mobility, which is number density or pressure dependent. Ifcurrent is limited by a regulator or ballast, then the voltage acrossthe gap will increase until the capsule body has come into thermalequilibrium.

In the operating mode, the arc discharge of the lamp generates a desiredlight output and a relatively low or moderate voltage occurs across theelectrodes of the lamp in response to a suitable arc discharge currentas established by the ballast or operating circuit related to the lamp.

The hot restrike mode occurs when the arc discharge of the lamp fails orextinguishes for some reason such as a momentary interruption of thecurrent supplied to the lamp. If the arc discharge extinguishesresulting in a loss in light output, the lamp may be permitted to coolfor a period of time before the arc condition can be restarted by therelatively high starting voltage.

Initiation of the early glow state during the hot restrike mode requireshigher external potentials because the total pressure within the lamp ismuch higher than during the cold starting mode. This higher gas pressureinhibits the sustained glow (i.e., Townsend glow). In general, the glow"time" will be longer since it is harder to build up the necessarycurrent density to form the thermionic mode.

During the eventual glow-to-arc mode when the pressure is high andparticularly while the metal halide salt is liquid, the cathode spotmode may terminate at locations other than the tip of the electrode. Thepersistence of arc formation at locations such as the press interface orliquid salt is very temporary since these regions cannot deliversustained electron flux. However, the arc spot can produce localizedheating which over repeated hot start times will cause detrimental lumenmaintenance.

A number of circuits have been developed in the past which specificallydeal with the problem of restriking various HID lamps while they are hotso as to avoid the temporary loss of light as discussed above. Othercircuits have been developed which simply wait a predetermined period oftime so that restarting can be accomplished after the lamp hascompletely cooled.

Some of the prior art circuits are unsuitable because they simply do notwork effectively, or are either relatively complex or are not reliable.More importantly, many of the these circuits are unsuitable for lowvoltage dc applications such as automotive headlights. In suchapplications, it is readily apparent that any delay in hot restarting anHID automotive headlight is intolerable.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to obviate thedisadvantages of the prior art.

It is still another object of the invention to provide a ballastsuitable for low dc voltage applications such as a ballast for HIDautomotive headlights.

It is another object of the invention to provide a ballast for improvingthe formation of the arc state and reducing sputtering of electrodematerial.

It is still another object of the invention to provide a ballast forreliably starting an HID lamp initially and quickly restarting the lampwhen it is hot after an interruption in lamp current.

These objects are accomplished, in one aspect of the invention, by theprovision of a ballast for operating an arc discharge lamp (e.g., an HIDlamp) in a dc mode. The ballast comprises a pulse width modulatorincluding a high frequency generator means and semiconductor switchmeans (e.g., a pair of field-effect transistors) coupled to first andsecond direct current input terminals. The high frequency generatordevelops a signal for driving the semiconductor switch means. Theprimary winding of a first transformer is coupled to the semiconductorswitch means. A rectifier and voltage multiplier circuit is coupled tothe secondary winding of the first transformer for conversion of thevoltage appearing thereacross to a direct current output voltage havinga value, for example, from 500 to 700 volts.

The ballast further includes a first energy storage circuit coupled tothe rectifier and voltage multiplier circuit for storing energy forgenerating a high voltage spike sufficient for initiating a glowcondition in the lamp. An output or pulse transformer for applying thehigh voltage spike to the lamp is coupled to the first energy storagecircuit. A second energy storage circuit coupled to the rectifier andvoltage multiplier circuit stores energy effective in causing aglow-to-arc transition in the lamp. A control signal developed by avoltage and current sensing circuit is coupled to the high frequencygenerator.

In accordance with further aspects of the present invention, therectifier and voltage multiplier circuit comprises a full wave bridgerectifier and a voltage doubler.

In accordance with further teachings of the present invention, the firstenergy storage circuit comprises a capacitor, charging resistor and aspark gap. Preferably, the second energy storage circuit comprises aseries connected capacitor, resistor and blocking diode.

In accordance with still further aspects of the present invention, thewindings of the output transformer are adapted to provide anegative-going spike across the lamp, i.e., opposite to the applied d.c.voltage across the lamp.

Additional objects, advantages and novel features of the invention willbe set forth in the description which follows, and in part will becomeapparent to those skilled in the art upon examination of the followingor may be learned by practice of the invention. The aforementionedobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combination particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent from the followingexemplary description in connection with the accompanying drawings,wherein:

FIG. 1 is a block diagram illustrating the basic form of an improvedstarting and operating ballast for an arc discharge arc lamp inaccordance with the present invention; and

FIG. 2 is a circuit diagram of a specific embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawings.

Referring to FIG. 1, there is illustrated a block diagram showing thebasic form of an improved ballast suitable for use with at least one arcdischarge lamp 22. A pulse width modulator (PWM) 10 connected to a lowvoltage dc power source (i.e., 12 volts) provides a periodic signal ofpredetermined frequency (e.g., 20 KHz.) to drive the primary winding ofa step-up transformer 12. The ac output voltage appearing at thesecondary winding of transformer 12 (e.g., 300 volts) is converted to ad.c. signal and increased to, for example, 500-700 volts by a rectifierand voltage multiplier circuit 14. Both lamp current and output voltageare sensed by circuit 16 and a resulting signal is delivered to PWM 10in order to permit the ballast to function as a fixed power supply. Highvoltage energy for lamp ignition and glow-to-arc transition is stored incircuit 18 and delivered to the primary winding of an output transformer20 which provides a high voltage ignitor spike (e.g., 25-30 KV) to arcdischarge lamp 22.

Reference is made to FIG. 2 which illustrates a detailed schematic of apreferred embodiment of a circuit for starting and operating an arcdischarge lamp according to the present invention. Lamp 22 may be aminiature, low-wattage, metal halide HID lamp suitable for use as anautomotive headlight. Typically, the lamp wattage of such lamps is fromabout 30-35 watts. Lamp 22 includes an envelope (i.e., quartz)containing an anode and a cathode (not shown). A fill material includingsodium, scandium, mercury, iodine and xenon at a pressure ofapproximately 8 atmospheres (cold) may be contained within the envelope.After the lamp has been operated for a period of time, the pressurewithin the envelope may be as high as 80 atmospheres and the walltemperature may be 1000 degrees Celsius. These lamp conditions arepartly responsible for the difficulty in attempting to hot restart thelamp.

The starting and operating circuit includes a pair of input terminalsIN1, IN2 formed for connection to a dc voltage source from about 10-16volts dc. Positive input terminal IN1 is coupled to pulse widthmodulator 10 and to one end of a filter capacitor C0 by means of asafety fuse F1. It is understood that the starting and operating circuitcan be operated from a source of ac voltage by coupling input terminalsIN1, IN2 to the output of circuitry suitable for converting the acsource voltage to dc.

Pulse width modulator 10 includes a high frequency generator IC1 whichproduces two squarewave signals Φ1, Φ2 which are 180 degrees out ofphase with each other at pins 11 and 14. Each signal Φ1, Φ2 has anadjustable duty cycle. The bi-phase signals Φ1, Φ2 are respectivelycoupled through resistors R3, R4 to the gates of transistors Q1, Q2. Thefrequency of the bi-phase signals is predetermined by the properselection of resistor R5 and capacitor C4. One suitable frequency is 50KHz. A resistor R6, connected between pin 5 and the discharge pin 7 ofIC1, provided dead time adjustment. Pins 15, 8, 9, 16 on IC1 arerespectively coupled to IN2 (i.e., ground) by means of capacitors C3,C5, C6 and C7.

A voltage divide network comprising resistors R8, R9 and R10 isconnected between pin 16 of IC1 and ground. Variable resistor R9 of thevoltage divider network has a wiper terminal connected to pin 2 of IC1.Lamp wattage and, consequently, light output are controlled by theproper adjustment of resistor R9.

In a preferred embodiment, transistors Q1 and Q2 aremetal-oxide-semiconductor (MOS) type field-effect transistors (FETs).Transistors Q1, Q2 have a zener diode D1, D2 respectively connectedacross their gate and source terminals. The source terminals oftransistors Q1, Q2 are connected to ground through input terminal IN2. Aseries connection consisting of R1, C1 and R2, C2 is respectivelyconnected across the drain and source terminals of transistors Q1 andQ2. The drain terminal of driver Q1 is coupled to one end of the primarywinding W1 of a step-up transformer T1. Similarly, the drain terminal oftransistor Q2 is connected to one end of the primary winding W2 ontransformer T1. Transformer T1 has a center tap coupled to inputterminal IN1 through safety fuse F1.

Transistors Q1, Q2 alternately apply 12 volts dc to the primary windingsW1, W2 of transformer T1 causing an ac squarewave of voltage ofapproximately 300 volts to appear at the secondary winding W3 oftransformer T1.

In a presently preferred embodiment, transformer T1 has a ferrite coreof the type which is of the material which is designated #3C6A byPhilips Corp. Each of the transformer primary windings W1, W2 consist of8 turns bifilar of 3 each #25 wire. The secondary winding W3 oftransformer T1 consists of 140 turns of #30 wire.

The secondary winding W3 of transformer T1 is connected to a full wavebridge rectifier section D5, D6, D7, D8 of rectifier and voltagemultiplier circuit 14. The ac output voltage across winding W3 isconverted to a dc signal by the diodes D5, D6, D7, D8 and increased to500 to 700 volts dc by a voltage multiplier (e.g., a doubler) sectioncomprising capacitors C10, C11, resistors R19, R20 and diodes D9, D10.This dc voltage, which appears at the junction of diodes D9 and D10,provides a high open circuit voltage to lamp 22 suitable for efficientglow-to-arc conversion during the ignition stage of operation. A secondvoltage doubler comprising capacitors C10, C11, C12, resistors R21, R22,and diodes D12, D13 provides a source to charge a high energy storagecapacitor C13 of an energy storage circuit 18A. Energy storage circuit18A, which consists of a series connection of capacitor C13, resistorR23 and a blocking diode D11, provides a source of high voltage (e.g.,500-700 volts ), high initial current (e.g., 2 to 6 amps) energynecessary for glow-to-arc transition.

It has been discovered that providing a substantial momentary increasein the initial current pulse at ignition (especially during hotrestrike) improves the formation of the arc state (i.e., glow-to-arctransition) and thereby reduces sputtering of electrode material. Inmost cases, this means that the glow-to-arc current is larger than thesteady state current that is reached as the discharge warms up to normaloperating power. For example, for a lamp having a steady state lampcurrent of 0.33 amp, the glow-to-arc current pulse is 2.0 amps.

The output of rectifier and voltage multiplier circuit 14 is alsocoupled to circuit 18D for high voltage spike generating. Preferablycircuit 18D comprises a spark gap SG, one end of which is connected tothe junction of the series connected charging resistor R24 and capacitorC14. The other end of spark gap SG is connected to a pulse winding W4 ontransformer T2. When the power is first applied to input terminals IN1,IN2, capacitor C14 charges through resistor R24 until the breakdownvoltage of spark gap SG is reached. Spark gap SG may have a breakdownvoltage of, for example, 350 volts. When the breakdown voltage isreached, spark gap SG conducts to complete the loop including capacitorC14 and winding W4 on transformer T2. The energy in capacitor C14discharges through spark gap SG and pulse winding W4. The pulse voltageis increased by windings W5 and W6 of T2 to a value of approximately25-30 KV and applied across the anode and cathode terminals of arcdischarge lamp 22. This 25-30 Kv spike causes a glow state in lamp 22. Atrain of these high voltage spikes is produced only until lamp ignitionoccurs since the voltage developed across capacitor C14 during normallamp operation is insufficient to cause further breakdown of spark gapSG.

While a positive high voltage spike will initiate a glow in the lamp, insome dc-operated high pressure discharge lamps it has been discoveredthat such a spike, which is added to the open circuit voltage, mayattack the quartz region of glass around the cathode. Consequently,windings W4, W5 and W6 are phased as illustrated in FIG. 2 so that anegative-going (i.e., opposite to the applied dc voltage across thelamp), high voltage spike is developed across lamp 22. Thenegative-going spike insures that the electrons properly migrate fromthe cathode to the anode. Moreover, the use of the negative-going highvoltage spike improves hot restriking.

In accordance with further teachings of the present invention, theinductance of windings W5 and W6 of transformer T2 advantageously limitsand shapes the lamp current during the glow-to-arc transition. Moreover,this inductance improves the current waveform ripple during lampoperation. Using the secondary windings W5, W6 for both voltageamplification and some inductive current regulation, contributes towardminimizing parts and optimizing the performance of the circuit.

In a presently preferred embodiment, trigger transformer T2 has aferrite core of the type which is of the material which is designated#77 by Fair-rite Prod. Corp. and has two 0.060 inch gaps. The primarywinding W4 of transformer T2 consists of 8 turns of #28 wire. Each ofthe secondary windings W5, W6 consist of 300 turns of #28 wire.

Both output voltage and lamp current are sensed and the resultingcomposite signal is delivered to the PWM in order to cause the ballastto function as a substantially fixed power control over the lamp voltagerange from 70-100 volts. More specifically, the voltage measured at theoutput of the full wave bridge rectifier D5-D8 in the rectifier andvoltage multiplier circuit 14 is sensed by an output voltage sensingcircuit 16A which also provides limits during various modes of lampoperation. For example, when the voltage across the lamp is greater thanabout 150 volts i.e., during hot restrike and initially during coldstart, a voltage signal delivered to pin 1 of IC1 causes maximum current(e.g., 0.6 amp) to be available to the lamp.

Output voltage sensing circuit 16A includes a bipolar transistor Q3having a base coupled to the output of bridge rectifier D5-D8 (at thejunction of diodes D5 and D6) by means of the series combination ofzener diode D4 and resistor R17. The emitter of transistor Q3 isconnected to ground. A bias resistor R18 is connected across thebase-emitter junction of transistor Q3. The junction of zener diode D4and resistor R17 is coupled to pin 16 of IC1 via the series connectionof resistor R16 and diode D3. A parallel combination of R15 and C8 isconnected across the collector-emitter terminals of transistor Q3. Also,the collector terminal of transistor Q3 is coupled to pin 1 of IC1 bymeans of the series connection of resistor R13 and diode D14. Acapacitor C9 is connected from the junction of diode D4 and resistor R17to ground.

Lamp current is monitored by current sensing circuit 16B. In thepreferred embodiment in FIG. 2, current sensing circuit 16B consists ofa resistor R11 connected between input terminal IN2 and the junction ofwindings W4 and W6 on the transformer T2.

A feedback signal, which is derived by adding the voltage signals fromcircuits 16A and 16B, is delivered to pin 1 of IC1. As a result, theduty cycle of the bi-phase signals Φ1, Φ2 decreases with increases inlamp current. Similarly, decreases in lamp current cause an increase induty cycle of the bi-phase signal.

The operation of an HID lamp on the starting and operating circuit ofthe present invention will now be described. At cold start and hotrestart, the open circuit voltage across the lamp gradually increases to500-700 volts dc. When capacitor C14 charges to 350 volts (as determinedby the breakdown voltage of spark gap SG), a high voltage spike ofapproximately 25-30 KV (perferably negative going with respect to theopen circuit voltage) is superimposed on the open circuit voltage and isappled across the lamp electrodes. The energy of the high voltage spikeis sufficient to produce a glow condition within the lamp.

Within less than approximately 100 usec from the time of the highvoltage spike, the energy stored in capacitor C13 begins to dischargethrough lamp 22 causing a rapid glow-to-arc transition. In a cold startmode of operation, the circuit provides a current pulse of approximately2 amps. Thereafter, the output voltage and lamp current sensing circuit16 causes the open circuit voltage to drop to approximately 20 volts andthe lamp current to be approximately 0.6 amps. A 30-watt lamp willoperate at approximately 18 watts until the lamp warms up and the lampvoltage increases to about 50 volts. At the 50-volt level, the outputvoltage and lamp current is regulated so that the product of current andvoltage equals approximately 30 watts. During the steady state operatingmode, the lamp voltage is approximately 90 volts and the lamp current isapproximately 0.33 amp.

As a specific example but in no way to be construed as a limitation, thefollowing components are appropriate to an embodiment of the presentdisclosure, as illustrated by FIG. 2:

    ______________________________________                                        Item      Description    Value                                                ______________________________________                                        R1        Resistor       22 ohm, 1/2 w                                        R2        Resistor       22 ohm, 1/2 w                                        R3        Resistor       15 ohm                                               R4        Resistor       15 ohm                                               R5        Resistor       5.6 Kohm                                             R6        Resistor       100 ohm                                              R7        Resistor       56 Kohm                                              R8        Resistor       4.7 Kohm                                             R9        Resistor       500 ohm Pot.                                         R10       Resistor       4.7 Kohm                                             R11       Resistor       1.0 ohm, 1/2 w                                       R12       Resistor       4.7 Kohm                                             R13       Resistor       33 Kohm                                              R14       Resistor       3.9 Kohm                                             R15       Resistor       27 Kohm                                              R16       Resistor       47 Kohm                                              R17       Resistor       330 Kohm, 1/2 w                                      R18       Resistor       4.7 Kohm                                             R19       Resistor       1 Kohm, 1 w                                          R20       Resistor       1 Kohm, 1 w                                          R21       Resistor       33 Kohm, 1/2 w                                       R22       Resistor       33 Kohm, 1/2 w                                       R23       Resistor       270 ohm, 1/2 w                                       R24       Resistor       1 Mohm, 1/2 w                                        C0        Capacitor      2200 MFD, 16 v                                       C1        Capacitor      0.01 MFD, 100 v                                      C2        Capacitor      0.01 MFD, 100 v                                      C3        Capacitor      1.0 MFD, 50 v                                        C4        Capacitor      0.0022 MFD, 50 v                                     C5        Capacitor      0.1 MFD, 50 v                                        C6        Capacitor      330 PFD, 500 v                                       C7        Capacitor      0.1 MFD, 50 v                                        C8        Capacitor      0.1 MFD, 50 v                                        C9        Capacitor      0.1 MFD, 50 v                                        C10       Capacitor      330 PFD, 500 v                                       C11       Capacitor      330 PFD, 500 v                                       C12       Capacitor      330 PFD, 500 v                                       C13       Capacitor      0.1 MFD, 500 v                                       C14       Capacitor      0.33 MFD, 500 v                                      D1        Zener Diode    1N4744A, 15 v                                        D2        Zener Diode    1N4744A, 15 v                                        D3        Diode          1N4148                                               D4        Zener Diode    1N5254B, 27 v                                        D5        Diode          BYV26C                                               D6        Diode          BYV26C                                               D7        Diode          BYV26C                                               D8        Diode          BYV26C                                               D9        Diode          BYV26C                                               D10       Diode          BYV26C                                               D11       Diode          BYV26C                                               D12       Diode          BYV26C                                               D13       Diode          BYV26C                                               D14       Diode          1N4752A                                              F1        Fuse           7 amp                                                IC1       Integrated Cir.                                                                              UC3525AN                                             Q1        Transistor     IRF540                                               Q2        Transistor     IRF540                                               Q3        Transistor     2N3904                                               SG        Spark gap      CG2, 350L                                            ______________________________________                                    

There has thus been shown and described an improved circuit for HIDlamps. The circuit reliably starts the lamp initially and quicklyrestarts the lamp when it is hot after a power interruption or the like.The circuit is suitable for use in low dc voltage applications such as aballast for HID automotive headlights. Also, the circuit of the presentinvention improves the formation of the arc state and reduces sputteringof electrode material.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention.Therefore, the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of theinvention. The matter set forth in the foregoing description andaccompanying drawings is offered by way of illustration only and not asa limitation. The actual scope of the invention is intended to bedefined in the following claims when viewed in their proper perspectivebased on the prior art.

What is claimed is:
 1. A ballast for operating an arc discharge lamp in a dc mode, said ballast comprising:first and second direct current input terminals; pulse width modulator means coupled to said first and second direct current input terminals and including high frequency generating means and semiconductor switch means, said high frequency generating means developing a signal means for driving said semiconductor switch means; first transformer means having primary and secondary windings, said primary winding being coupled to said semiconductor switch means; rectifier and voltage multiplier means coupled to said secondary winding of said first transformer means for conversion of the voltage across aid secondary winding of said first transformer means to a direct current output voltage having a predetermined value; first energy storage means coupled to said rectifier and volage multiplier means for storing energy for generating a high voltage spike sufficient for initiating a glow condition in said lamp; second transformer means coupled to said first energy storage means for applying said high voltage spike to said lamp; second energy storage means coupled to said rectifier and voltage multiplier means for storing energy for application to said lamp, said second energy storage means being effective in causing a glow-to-arc transition in said lamp; and voltage and current sensing means for developing a control signal and means for coupling said control signal to said high frequency generating means.
 2. The ballast of claim 1 wherein said high frequency generating means developing a pair of signals for respectively driving a pair of semiconductor switches.
 3. The ballast of claim 1 wherein said semiconductor switch means comprises a pair of field-effect transistors.
 4. The ballest of claim 1 wherein said first transformer means includes a step-up transformer having first and second priamry windings coupled respectively to a pair of semiconductor switches.
 5. The ballest of claim 1 wherein said predetermined value of said direct current output voltage is from 500 to 700 volts.
 6. The ballast of claim 1 wherein said rectifier and voltage multiplier means comprises a full wave bridge rectifier and voltage doubler.
 7. The ballast of claim 1 wherein said first energy storage means comprises a series connected capacitor and charging resistor, and a spark gap connected to the junction of said series connected capacitor and charging resistor.
 8. The ballast of claim 1 wherein said second energy storage means comprises a series connected capacitor, resistor and blocking diode.
 9. The ballast of claim 1 wherein said second transformer means includes an output transformer having a primary winding coupled to said first energy storage means and first and second secondary windings coupled to said lamp.
 10. The ballast of claim 9 wherein said primary winding and said first and second secondary windings of said output transformer are adapted to provide a negative-going spike across said lamp.
 11. The ballast of claim 9 wherein one end of each of said first and second secondary windings of said output transformer is coupled to a respective end of said lamp.
 12. The ballast of claim 1 wherein said at least one lamp is a single high intensity discharge lamp.
 13. A ballast for operating a high intensity discharge lamp in a dc mode, said ballast comprising:first and second direct current input terminals; pulse width modulator means coupled to said first and second direct current input terminals and including high frequency generating means and semiconductor switch means, said high frequency generating means developing a signal means for driving said semiconductor switch means; first transformer means having primary and secondary windings, said primary winding being coupled to said semiconductor switch means; rectifier and voltage multiplier means coupled to said secondary winding of said first transformer means for conversion of the secondary winding voltage to a direct current output voltage having a predetermined value; first energy storage means coupled to said rectifier and voltage multiplier means for storing energy for generating a high voltage spike sufficient for initiating a glow condition in said lamp; second transformer means coupled to said first energy storage means and adapted to apply across the lamp said high voltage spike in a negative-going direction with respect to said output voltage; second energy storage means coupled to said rectifier and voltage multiplier means for storing energy for application to said lamp, said second energy storage means being effective in causing a glow-to-arc transition in said lamp; and voltage and current sensing means for developing a control signal and means for coupling said control signal to said high frequency generating means. 